Determine the optimum preventive maintenance frequency for each of the pieces of equipment if breakdown time is normally distributed: $($Round "Probability Ratio" to $4$ decimal places, and all other answers to $2$ decimal places. Negative values should be indicated by a minus sign.$)$

Use Table.

Equipment $|$ Average Time $($days$)$ between Breakdowns $|$ Standard Deviation

A$201203$

B$400303$

C$850455$

Equipment $|$ Preventive Maintenance Cost Breakdown $|$ Cost

A$201$ $$330$ $$2,230$

B$400$ $$230$ $$3,530$

C$850$ $$580$ $$4,825$

Click here for the Excel Data File. Find the probability ratio, z and interval $($days$)$ for A$201,$ B$400$ & C$850.$